1. Field of the Invention
This invention relates to a parts feeding apparatus of the piezoelectric drive type which employes piezoelectric elements as a drive source of a conveying means for feeding relatively small machine parts, electric elements or the like.
2. Description of the Prior Art
Parts feeders employing piezoelectric elements as a drive source are disclosed by Japanese Utility Model Japanese Utility Model Unexamined Application Nos. 52-61087 and 57-46517. With reference to FIG. 20 illustrating the general construction of such prior art parts feeder of the piezoelectric drive type, lower ends of two leaf springs 4 are connected with ends of a lower frame 2 fixed to a base 1. Upper ends 4a of the leaf springs 4 are connected with an upper frame 3. The leaf springs 4 are inclined and parallel to each other. A trough 5 as a conveying means is mounted on the upper frame 8 and article 6 such as machine parts to be conveyed are placed thereon. Two rectangular piezoelectric elements 7a and 7b are provided on respective sides of each leaf spring 4. AC voltage supplied to terminals 8 and 9 is applied to the piezoelectric elements 7a and 7b through leads 8a and 9a.
Each bimorph cell assembly 10 as a vibration generator thus comprises a leaf spring and two piezoelectric eIements 7a and 7b fixed thereto. When AC voltage is applied to each of the piezoelectric elements 7a and 7b, one piezoelectric element 7a expands in a positive half cycle and contracts in a negative half cycle to thereby induce strain movement. The other piezoelectric element 7b is adapted to contract in the positive half cycle and expand in the negative half cycle. For example, where two piezoelectric elements 7a and 7b are fixed to the leaf spring 4 so as to have opposite polarity to each other at outer sides thereof and further where the leads 8a and 9a are wired such that AC voltage having the same polarity is applied to each of the piezoelectric elements 7a and 7b, one piezoelectric element 7a expands while at the same time the other piezoelectric element 7b contracts. The strain movement of the piezoelectric elements 7a and 7b causes the flexure movement to the leaf spring 4 to vibrate it in the direction of arrow 11 in FIG. 20, thereby vibrating the trough 5.
The parts feeder of the piezoelectric drive type is smaller in size and simpler in construction than those of electromagnetic drive type and electric motor drive type, so that it provides easy operation and repair. Furthermore, the piezoelectric drive type parts feeder surpasses those of the other types in consumption of electricity and noise. On the other hand, it has some problems on the point of feeding efficiency: In the above-described piezoelectric drive type parts feeder, application of AC voltage to the piezoelectric elements 7a and 7b causes the elements 7a and 7b to bend with the leaf spring 4, which movement vibrates the trough 5 connected with the free end 4a of each leaf spring 4 obliquely up and down as shown by arrow 12 in FIG. 20, thereby feeding the articles 6 in the direction of arrow 13 along the trough 5. In this case, the speed at which the articles are conveyed is proportionate to the vibration amplitude of the trough 5.
Reference symbol ".delta." in FIG. 21 denotes displacement of each free end 4a of the leaf spring 4 (connection with the trough 5) when each bimorph cell assembly 10 suffers deformation by application of AC voltage thereto. The displacement .delta. is shown by the following expression (1): EQU .delta.=(3/2)(d.multidot.V/t.sup.2){+(.sigma./t)}l.sup.2 .multidot..alpha.(1)
where d=piezoelectric strain constant
V=voltage applied to piezoelectric elements PA1 t=thickness of bimorph cell assembly PA1 l=effective length of leaf spring PA1 .sigma.=thickness of leaf spring PA1 .alpha.=non-linear coefficient PA1 where E=width of bimorph cell assembly PA1 E=Young's modulus where the value of applied voltage is zero PA1 fn=vibration frequency PA1 .eta.=conveying efficiency
The amount of displacement of each bimorph cell assembly 10 is decreased when an external force acts on the free end 4a of each leaf spring 4 in the opposite direction to that of displacement of each bimorph cell assembly 10. The amount of displacement .delta. becomes zero when the external force reaches a bound load Fb shown by the following expression (2): EQU Fb=(1/4)(.omega..multidot.t.sup.2 /l.sup.3).sigma..multidot.E (2)
FIG. 22 illustrates measured results about the relation between displacement .delta.of the bimorph cell assembly 10 and the bound load Fb in the case where DC voltage (100 V) is applied to the piezoelectric elements 7a and 7b. It is known in the art that the resonance increases the amount of displacement .delta. by ten times or more than that in non-resonance when AC voltage applied to the piezoelectric elements 7a and 7b has the same frequency as the natural frequency of the bimorph cell assembly 10. The value of the bound load Fb, however, does not change, whether the resonant frequency is selected or not.
The vibration frequency rapidly decreases when a load acts on the bimorph cell assembly 10, so that it is necessary for the load not to be applied to the free ends 4a of the leaf spring 4.
3. Defects of the Prior Art
Since the bimorph cell assemblies 10 of the parts feeder shown in FIG. 20 have the same length and are placed in parallel to each other, the trough 5 is not allowed to be inclined even when each bimorph cell assembly 10 vibrates right and left. In this case, the trough 5 cannot but vibrate obliquely up and down, keeping its horizontal state. As a result, an external bending force acts on the connection where each leaf spring 4 is connected with the upper frame 3 supporting the trough 5. That is, in FIG. 21, the angle formed by each leaf spring 4 and the trough 5 necessitates changing from .theta..sub.o to .theta..sub.1 when application of voltage to the piezoelectric elements 7a and 7b moves each bimorph cell assembly 10 from the initial position shown by the solid line to a position shown by the chain line. If this angular change is not allowed, a bending stress as load acts on each bimorph cell assembly 10. When the bending stress increases above the bound load Fb, the trough 5 cannot be vibrated.
The speed V at which the articles 6 are conveyed depends on displacement .delta. of the trough 5 and is denoted by the following expression (8): EQU V=(.delta..times.fn).eta. (3)
where
Even when the resonant frequency is selected as the vibration frequency fn, the vibration amplitude is decreased with increase of load applied to the free ends 4a of the leaf springs 4 as shown in FIG. 23.
In the prior art parts feeder of the piezoelectric drive type, each leaf spring 4 has high stiffness in the portion between the point where the trough 5 is connected with the leaf spring 4 (point P1 in FIG. 20) and the upper end of each of the piezoelectric elements 7a and 7b (point P2 in FIG. 20). Accordingly, a large force acts on each bimorph cell assembly 10 in vibration. Consequently, the vibration amplitude of each bimorph cell assembly 10, that is, that of the trough 5 is decreased, which makes it impossible to obtain the conveying speed for practical use. The prior art parts feeder thus necessitates improvements: the load acting on each bimorph cell assembly 10 needs decreasing and the vibration amplitude of each bimorph cell assembly 10 needs increasing. Furthermore, a means is required for effectively transmitting the vibration with increased amplitude from each bimorph cell assembly to the trough.